1. Field of the Invention
The present invention relates to gas sterilizable packages and more particularly to fiber free tear strips which are used to cover the access opening of the bag portion of such packages.
2. Prior Art
Gas sterilizable packages for maintaining the sterility of objects which are to be used, for example, in surgery, has been available for many years. Typically such packages utilize a bag fabricated from a flexible non-porous plastic such as polyethylene sealed to prevent the entry of bacteria. One portion of the bag is left open and is covered by a gas permeable membrane, i.e., a material which has pores small enough to prevent the passage of bacteria, but large enough to allow a sterilizing gas such as ethylene trioxide (ETO) to pass. There are a number of surgical grade papers and some plastic materials, for example, a DuPont product called Tyvek, which are suitable for this purpose. Sterilization is accomplished after sealing by exposure of the sealed package to the sterilizing gas. A typical prior art gas sterilizable package is described in U.S. Pat. No. 3,754,700 issued to Bonk.
The most convenient and practical embodiments of gas sterilizable packages being manufactured today use the gas permeable membrane as a cover for the access opening of the bag. That is, the gas permeable membrane is peelably adhered to the bag around the opening through which the contents are to be removed. When the bag contents are to be removed, the membrane, called a tear strip, is peeled off the bag and the contents spilled out or removed with tongs.
Sterilizable packages are typically fabricated from a continuous strip or strips of bag material on an intermittent feed heat sealing machine. The tear strip is sealed over the access opening at the same time as the other required seals are made. The bag is then cut from the supply strip leaving one seal unmade. At a later time the objects to be stored are inserted in the bag and the final seal made.
The sterilization process involves alternate cycles of pressure and vacuum using a sterilizing gas such as ETO which kills any bacteria inside the package. Since the bag itself is pinhole free and the tear strip is impervious to bacteria, the package can be stored in a non-sterile environment, yet the contents remain sterile.
The problem which has faced gas sterilizable package designers heretofore is that the papers commonly used for tear strips are easily abraded, and the access opening becomes contaminated with paper fibers, and with many bag designs, fibers even find their way into the bag. This problem has been alleviated in the past by either coating the tear strip with a peelable adhesive or calendering the paper so that it has a hard tight surface. Unfortunately, both of these methods impair the breathability of the paper. A typical coating will slow the breathing rate by a factor of three. A relatively heavy coating may be necessary since usually the faster the breathing rate the weaker the seal strength of the package.
The common measure of breathing rate is the number of seconds required for 100 cc of air to pass through one square inch of material. Uncoated papers have a breathing rate of less than 50 seconds, but when coated, the breathing rate may range from 100 to 300 seconds.
Until this invention, the packaging engineer had to choose the coating that would give the proper seal strength for the weight and bulk of the product and then would have to choose a sterilization cycle that would insure the proper penetration of the gas.
The packaging engineer was constantly faced with a compromise, if he had high seal strength that would insure that the product would be still sterile by the end use, he might not have a properly sterilized product because the gas has a hard time penetrating the coating. The cycle times for the sterilization process can range from as little as 8 hours for very fast breathing packages to over 24 hours for slow breathing packages. If the engineer chose a high seal strength, thus a slow breathing package, the sterilization cost can run three times as much. A third problem arises with a slow breathing package and that is residual sterilization gas staying inside the package. Small amounts of the gas will attach to the product unless the gas is vigorously purged through rapid pressure changes. These rapid pressure changes cause even more stress on the seals. An ideal tear strip for a gas sterilized medical product package would have a breathing rate of under 50 seconds with a seal strength of over 1 lb per inch of seal, and provide a clean fiber free sterile access opening for the product to be withdrawn from.
It is therefore an object of the present invention to provide a rapid breathing tear strip for a gas sterilizable package which has a high speed strength.
It is a further object of the present invention to provide a fiber free tear strip for a gas sterilizable package.
Other objects and advantages of the present invention will become apparent from the following specification and the drawings.